1. Field of the Invention
This invention relates to a single tube color television camera comprising a picture tube and two color strip filters of different color filter effect which are located one behind the other in the optical image axis and are either installed within the picture tube or are structurally separated therefrom, and more particularly to such a color television camera in which the color strip filters are oriented at the same angle with respect to the vertical image axis, but in opposite directions, and in which the camera comprises a decoding arrangement containing a comb filter for splitting the output signal of the picture tube.
2. Description of the Prior Art
Color strip filters are known in the art. For example, the German Offenlegungsschrift No. 1,956,940 discloses color strip filters consisting of strips of color selective filter material and non-selective light transmissive material disposed in an alternate arrangement of parallel strips. The color selective strips of two color strip filters which are disposed one behind the other select different color components for each filter. Therefore, a modulation of the luminance information scanned by the electron beam of the picture tube is created in that together the filters contain the entire chrominance information. The chrominance information consists of two parts which can be distinguished in that the strips of one filter are arranged basically at a right angle to the scanning lines, that is parallel to the vertical image axis, and those of the other filter are arranged at an angle with respect to the vertical image axis.
In the periodical "Proceeding of the IEE," February 1971, at Pages 322 to 323 there is a discussion of how the chrominance signal is composed as created by means of the aforementioned known color strip filter. It is thereby recognized in quality that, on the one hand, both chrominance informations have spectrums of frequencies which amount to a multiple of the line frequency and that, on the other hand, the spectral frequencies which originate from the color strip filter so oriented with respect to the vertical image axis are shifted in frequency in comparison to the spectral frequencies which originate from color strip filters which are not so angularly oriented, which means that both spectrums are joined with each other. However, also the luminance information is contained in the spectrum of the integral multiple of the line frequency. By means of a suitable choice of the color selection of the color strip filters, a summation spectrum is obtained which is composed of a spectrum of the integral multiple of the line frequency containing, in a lower frequency range, the green information and, in a higher frequency range, the blue information, and of a spectrum of frequencies located therebetween containing the red information and being basically designed between the other two regions. By means of a comb filter, both spectrums can be separated from each other. The separation of the blue and green color informations from the same spectrum must be carried out by means of low pass filters and high pass filters, respectively. Therefore, blurriness in the separation cannot be avoided.
It is further known from Larsen et al, U.S. Pat. No. 3,781,519, granted Aug. 1, 1972 and corresponding to the German Offenlegungsschrift No. 2,163,443, to orient both color strip filters in respect of the vertical image axis by the same angle of rotation, however in a different direction of rotation. Again, the separation of the spectral frequencies takes place by means of a comb filter. A clean separation of the chrominance signals from the luminance signal, however, poses certain difficulties. In order to avoid Moire patterns it is suggested in this published application to filter out those frequencies which cause Moire patterns and erroneous color information with optical components. An optical phase refraction lattice is suggested for alignment so that the undesired frequencies are eliminated. However, a substantial disadvantage is achieved, namely that the luminance information is considerably limited.